1. Field of the Invention
The present invention relates to an engine including a secondary air supply apparatus for supplying secondary air to an exhaust gas passage in a cylinder head.
2. Description of the Related Art
As shown in the first to the third conventional examples of FIGS. 9 to 11, in an engine including a secondary air supply apparatus, usually secondary air is taken from an air cleaner, and the secondary air is supplied to an exhaust gas passage through secondary air passages having various configurations.
In the first conventional example shown in FIG. 9, a linearly drill hole is made as a secondary air passage 103 above an exhaust gas passage 101 in a cylinder head 100, and an inlet 103a of the secondary air passage 103 is opened at an upper portion on an outer peripheral surface 100a of the cylinder head 100 on to which an exhaust gas outlet 101a of the exhaust gas passage 101 is opened. A secondary air pipe 104 is connected to the inlet 103a. The secondary air pipe 104 is extended toward an air cleaner 106 provided on the opposite side of the exhaust gas outlet, and the secondary air pipe 104 is connected to the air cleaner 106 through a secondary air lead valve 105. Assuming that the side of the outer peripheral surface 101a of the cylinder head 100 is set to “forward” as shown by an arrow, the secondary air passage 103 is rear downwardly extended from the inlet 103a to an outlet opened at an upper part of the exhaust gas passage 101 such that the secondary air passage 103 is in communication with the exhaust gas passage 101. For example, the first conventional engine has a four-valve structure including a pair of intake valves and a pair of exhaust valves in one cylinder, an exhaust upstream portion of the exhaust gas passage 101 is branched into two by a partition 101b, and the secondary air passage 103 is in communication from above with an exhaust downstream portion where the two branch exhaust gas passages are merged in the exhaust gas passage 101.
In the second conventional example shown in FIG. 10, the secondary air lead valve 105 is attached to the outer peripheral surface 100a of the cylinder head 100 on which the exhaust gas outlet 101a of the exhaust gas passage 101 is opened. In FIG. 10, the same component as that of FIG. 9 is designated by the same numeral or number. The secondary air lead valve 105 is disposed above the exhaust gas outlet 101a. The secondary air passage 103 is rear downwardly extended in the cylinder head 100 from the lead valve 105, the secondary air passage 103 reaches the partition 101b of the exhaust gas passage 101, and is in communication with each branch exhaust gas passage on both sides of the partition 101b through a lateral passage portion 108.
In the third conventional example shown in FIG. 11 is disclosed in Japanese Unexamined Patent Application Publication No. 2003-74319, the lead valve 105 is attached to an upper end of a head cover 110 fastened to an upper end of the cylinder head 100. In FIG. 11, the same components as that of FIG. 9 are designated by the same numerals. Secondary air passages 111 and 103 are formed in the head cover 110 and the cylinder head 100 respectively, and the secondary air passages 111 and 103 are in communication with the upper part of the exhaust gas passage 101.
In the first conventional example shown in FIG. 9, when the secondary air passage 103 is drilled in the cylinder head 100, it is required to make a long stroke hole from the inlet 103a to the exhaust gas passage 101 using a long and thin drill. This hole making work becomes troublesome, because the hole making work should be performed such that the drill is not broken and such that the drill is not bent during the work. Further, given that re-burning of unburned gas is promoted by supplying the secondary air to a high-temperature point near a combustion chamber 109 of the engine, preferably the outlet of the secondary air passage 103 is disposed near the combustion chamber 109. However, when the outlet of the secondary air passage 103 is brought close to the combustion chamber 109, the secondary air passage 103 is lengthened, which makes the hole making work of the secondary air passage 103 with the drill more difficult. Further, since the secondary air passage 103 is formed above the exhaust gas passage 101 and the secondary air pipe 104 is passed through an upside of the engine from the inlet 103a and connected to the air cleaner 106, a height of the engine is increased in the structure.
In the structure of the second conventional example shown in FIG. 10, similarly to the first conventional example, it is necessary that the hole making work of the secondary air passage 103 be performed for a long stroke using a long and thin drill, so that a large amount of time is taken in forming the secondary air passage 103. Furthermore, the size in the forward and rearward direction of the engine and the height of the engine are increased because the secondary air lead valve 105 is disposed above the exhaust gas outlet 101a while attached to the outer peripheral surface (front wall surface) 100a of the cylinder head 100.
In the third conventional example shown in FIG. 11, the hole making work becomes troublesome because it is necessary to form the secondary air passages 103 and 111 in both the cylinder head 100 and the head cover 110. Additionally, the total height of the engine is increased because the secondary air lead valve 105 is attached to the upper end of the head cover 110.